1. Field of the Invention
The present invention relates to a quartz thermometer and, more particularly, to a thermometer in which the temperature-sensitive element is a quartz resonator which is driven by an oscillator circuit so as to produce an electrical signal, the frequency of which is representative of the temperature to which the resonator is subjected.
2. Description of the Prior Art
Temperature measuring devices which use a quartz resonator as the temperature-sensitive element are well known. It is also well known that the frequency of such a resonator varies with temperature. The relationship between the frequency of oscillation of quartz and temperature may be represented by the following polynomial expression: EQU f.sub.T =f.sub.To [1+.alpha.(T-To)+.beta.(T-To).sup.2 ]
where f.sub.To and To are constants, T is the temperature to be measured, f.sub.T is the frequency of the quartz oscillator subjected to temperature T, and .alpha. and .beta. are coefficients that are independent of temperature.
In general, it is not sufficient that the frequency of a quartz oscillator depends on temperature, it being very desirable that this dependence be as linear as possible. The main advantage of this type of thermometer, as compared with all other known devices, resides in the direct conversion of temperature to frequency, i.e. the information regarding temperature is presented in a quasi-numerical form. Another advantage of this type of thermometer is that the frequency is a physical quantity which can be measured with a degree of resolution as great as is desired if the measurement time is sufficient.
There are presently available on the market thermometers provided with a probe containing a vibrating quartz crystal, the frequency of which depends on temperature in a very linear manner. A first apparatus, made by Hewlett-Packard, uses an LC cut quartz crystal which is excited to a freuquency of 28 MHz. This instrument has a sensitivity on the order of 1000 Hz per .degree.C. Another apparatus, made by Tokyo Dempa, uses a YS cut quartz crystal excited to a frequency on the order of 10 MHz. This instrument has a sensitivity of the same order of magnitude as the Hewlett-Packard instrument.
On the other hand, these two high precision instruments are very expensive and the quartz resonators used as the sensitive elements thereof have the disadvantage of being very cumbersome, since they are produced by means of a technology which does not permit easy miniaturization. Moreover, their methods of manufacture are unsuitable for mass production. Furthermore, since they are arranged to vibrate at a frequency equal to or greater than 10 MHz, the electronic circuits which process the signals emitted by the quartz resonators consume a relatively high amount of electrical energy.